A small bump on the back can be caused by sebaceous cysts, molluscum contagiosum, viral warts and other reasons. 1. Sebaceous cysts: the distribution of sebaceous glands in the hair follicles is exuberant, which can cause blockage of the follicular gland ducts, and the accumulation of secretions from the glands will form a cystic structure, which can also lead to the growth of a small pimple on the back. 2. Molluscum contagiosum: it is a benign tumor formed by the proliferation of connective tissue, which can also lead to a small bump on the back. 3. Viral warts: human papilloma virus infection formed by common warts, flat warts and other diseases, will cause benign skin superfluous, can also lead to the back of a small bumps. The actual fact is that there are many other reasons why you should go to a regular hospital to get a checkup, and then follow the doctor’s instructions.